titanium dioxide hybrid material

Accepted Manuscript Enhanced Antifouling and Antimicrobial Thin Film Nanocomposite Membranes with Incorporation of Palygorskite/Titanium Dioxide Hybrid Material Tian Zhang, Zhiqiang Li, Wenbo Wang, Yong Wang, Baoyu Gao, Zhining Wang PII: DOI: Reference:

S0021-9797(18)31289-X https://doi.org/10.1016/j.jcis.2018.10.092 YJCIS 24247

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

28 September 2018 25 October 2018 28 October 2018

Please cite this article as: T. Zhang, Z. Li, W. Wang, Y. Wang, B. Gao, Z. Wang, Enhanced Antifouling and Antimicrobial Thin Film Nanocomposite Membranes with Incorporation of Palygorskite/Titanium Dioxide Hybrid Material, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.10.092

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Enhanced Antifouling and Antimicrobial Thin Film Nanocomposite Membranes with Incorporation of Palygorskite/Titanium Dioxide Hybrid Material Tian Zhang a

a,1

, Zhiqiang Li b,1, Wenbo Wang c, Yong Wang d, Baoyu Gao a,*, Zhining Wang a, *

Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental

Science and Engineering, Shandong University, Qingdao, Shandong, 266237, P.R. China b

Advanced Research Center for Optics, Shandong University, Qingdao, Shandong, 266237, P.R. China

c

Center for Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese

Academy of Sciences, Lanzhou, Gansu, 730000, P.R. China d

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering,

Nanjing Tech University, Nanjing, Jiangsu, 210009, P. R. China

Abstract: Palygorskite (Pal) is a kind of low-cost and environment-friendly natural nanoclay material with tubular structure and excellent hydrophilicity. TiO2 nanoparticles, especially anatase phase, have prominent photocatalytic bactericidal and organic pollutant decomposition activities. In this work, Pal and Pal/TiO2 nanocomposite were successfully embedded in the polyamide (PA) selective layer of the reverse osmosis (RO) membranes via interfacial polymerization. The tubular structure of Pal possesses a cross-sectional area of 0.37×0.63 nm2, which facilitates the selective transport of water molecules through PA layers. The water flux of Pal incorporated TFN membrane increases to approximately 40 l·m-2·h-1 at 16 bar, which is 1.6-fold higher than the reference TFC membrane. Meanwhile, the NaCl rejection is maintained at approximately

*Corresponding authors. E-mail addresses: [email protected] (Z. Wang), [email protected] (B. Gao) 1 These authors contributed equally.

1

98%. Although the Pal/TiO2 incorporated TFN membrane exhibited slightly lower flux (1.4-fold higher than TFC), the embedded Pal/TiO2 contributed to the antifouling and photocatalytic bactericidal capacities and the salt rejection maintained at an acceptable level of 98%, which are greatly desired in the membrane desalination and water reclamation processes. Keywords: palygorskite; TiO2; antifouling; antibacterial; thin film nanocomposite membrane 1. Introduction Reverse osmosis (RO) is one of the most important and widely recognized technologies for saline water desalination due to their easy operation, low operational costs and high salt rejection abilities [1, 2]. Nowadays, the thin film nanocomposite (TFN) membranes fabricated by incorporating an appropriate amount of nanofillers into the polyamide (PA) selective layer have received increasing attentions due to their enhanced separation characteristics compared with the conventional thin film composite (TFC) membrane [3-7]. Certain TFN membranes provide not only opportunities to overcome the permeability-selectivity trade-off but also advances to improve fouling resistance and bactericidal capacity. Clays are one of the ideal nanofillers for TFN membranes because of their high intercalation chemistry, inherent hydrophilicity and readily available property [8-10]. Generally, clays are silicates or alumina silicates, which are composed of silicon, aluminum or magnesium, oxygen and hydroxyl with various associated cations. The ions and hydroxyl groups are organized into 2D structures, which joined together and stacked on top of each other with variable interlayer distance to form the layered silicates frame work. Due to their unique structure and property, clay materials have been widely used in membrane water treatment. As an exciting candidate for fabrication of high-performance ultrafiltration (UF) membranes, clay materials, such as Pal, montmorillonites (MMTs), and halloysite nanotubes (HNTs) were added into mixed matrix membranes, which achieved an enhanced water flux and improved antifouling capacity [11-14].

2

Moreover, MMTs, HNTs and layered double hydroxide (LDH), an anionic clay, were also incorporated in PA layers to prepare high performance TFN membranes [15, 16]. Palygorskite (Pal) is a naturally available 1D rod-like silicate clay mineral with high surface area and a large number of hydrophilic surface groups [17-19]. Meanwhile its tubular structure with the cross-sectional area of 0.37×0.63 nm2 facilitates the transport of water molecules [20]. Recently, Pal was added into polyvinylidene fluoride (PVDF) UF membrane to improve abrasion resistance and water permeability [21]. Owing to the high water retention capacity, Pal was further intercalated into adjacent graphene oxide (GO) nanosheet or mixed poly(vinyl alcohol) (PVA) to prepare hybrid membranes, which possessed underwater superoleophobic and low oiladhesive surface, therefore leading to outstanding separation performance for various oil-inwater emulsion systems [22, 23]. In addition, the incorporation of Pal in TFN nanofiltration (NF) membranes demonstrated increased permeability and improved antifouling capacity without any compromise in salt rejections [20]. Besides, Pal is an efficient carrier to prepare nanocomposites with homogeneous dispersion of nanoparticles on its surface [24]. In this study, a photocatalytic antimicrobial agent, titanium dioxide (TiO2) nanoparticles [25-28], was synthesized on the surface of Pal nanorods (Fig. 1). Then TFN membranes were successfully prepared by incorporation of Pal or Pal/TiO2 nanocomposite in the Pa layers via interfacial polymerization. Compared with the reference TFC membrane, TFN membranes exhibited higher separation characteristics.

3

Fig. 1. The fabrication procedure and application of Pal/TiO2 incorporated RO membranes. 2. Experimental section 2.1. Materials Palygorskite clay (Pal) was kindly supplied by Jiangsu Jiuchuan Nano Technology (China). Triton X-100 was purchased from Beijing Solarbio (China). Titanium (IV) isopropoxide (TTIP, >95%) was obtained from Shanghai Macklin Biochemical (China). Polysulfone membrane was purchased from Pureach Tech. (China). m-Phenylenediamine (MPD, >99%) and 1, 3, 5benzenetricarbonyl trichloride (TMC, >98%) was obtained from Aladdin (China) and Tokyo Chemical Industry (Japan), respectively. Sodium chloride, acetic acid, ethanol, and other chemicals were purchased from Sinopharm Chemical (China). 2.2. Pal/TiO2 nanocomposite preparation The Pal/TiO2 nanocomposite was synthesized according to a previously reported procedure [29]. Briefly, TTIP was added to certain concentration Triton X-100 ethanol solutions under vigorous stirring. Subsequently, acetic acid was added for esterification reaction with ethanol. Hydrolysis of TTIP occurred with water produced by the above esterification reaction. Then appropriate amount of Pal powder was added into the resultant solutions with different Pal to

4

TiO2 mass ratios (Pal/TiO2 = 3/1, 5/1, 10/1, 20/1, 30/1, 50/1). The mixtures were calcined at 500 °C for 2 hours to remove the organic content. Finally, the Pal/TiO2 nanocomposite was prepared. 2.3. Membranes preparation The TFC membrane was prepared via interfacial polymerization. The aqueous solution containing 2 w/v% MPD and 0.15 w/v% sodium dodecyl sulfate (SDS) was poured onto the PSf support membrane for 2 min. Then, the redundant MDP solution was drained and then the membrane was placed in air to dry naturally. Subsequently, an organic phase containing 0.1 w/v% TMC/n-hexane was added onto the membrane surface and reacted with the remaining MPD solution for 1 min. After that the TMC solution was drained. The prepared membrane was dried in air for 1 min and in an oven at 80 °C for 5 min, respectively. The TFN membranes incorporated with Pal and Pal/TiO2 were prepared by adding certain amounts of Pal or Pal/TiO2 nanomaterials to the MPD solution. The Pal and Pal/TiO2 were well dispersed by ultra-sonication for 1 hour. The effect of different concentration and mass ratio of Pal/TiO2 nanocomposite on membrane performance was investigated. The as-prepared membrane was denoted as TFNx-Pal/TiO2(m/n), where x represented the mass concentration (mg L-1) and m/n represented the Pal to TiO2 mass ratio, respectively. For example, TFN75-1

Pal/TiO2(20/1) indicated that 75 mg·l Pal/TiO2 with a 20/1 Pal to TiO2 mass ratio was dispersed in the MPD aqueous phase to prepare the selective layer via interfacial polymerization. 2.4. Characterization The chemical properties of Pal and Pal/TiO2 nanocomposite were analyzed by Fourier transform infrared spectroscopy (FTIR, Bruker Tensor 27) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The crystalline structures of Pal and Pal/TiO2 were examined by X-ray diffraction patterns (XRD, Bruker D8 ADVANCE). A dynamic light scattering (DLS, Malvern Zetasizer Nano) instrument was used to analyze the zeta potential and particle size of Pal and

5

Pal/TiO2. The Raman spectra (Renishaw, UK) were acquired at 532 nm excitation and 10 mW. Scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, FEI TECNAI G2 F20) were utilized to investigate the morphology of Pal and Pal/TiO 2 nanocomposite. Before SEM and TEM measurements, the sample was sonically dispersed in water for 1 h. In addition to material characterization, membrane properties were also essential to be measured. The membranes were investigated by SEM (Hitachi SU8010, Japan) and energy dispersive X-ray spectroscopy (EDS) in order to analyze the surface morphology and element analysis. All samples were gold-sputtered for 50 s to improve their conductivity. Atomic force microscope was also used to analyze the surface morphology and roughness. The root-meansquare-height (Sq) roughness was obtained from three different spots of each membrane. The chemical structure of membranes was analyzed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Bruker Tensor 27, Germany). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) was used to examine the elemental composition of TFC and TFN membranes. Before FTIR and XPS measurement, all the samples were dried in a vacuum oven at 40 °C overnight. The water contact angle of the membrane surface was calculated by a drop shape analysis system (DSA 100, Kruss, Germany) at room temperature. A 5.0 mL droplet of DI water placed on the membrane surface was used to perform the measurement. The surface charge property of TFC and TFN membranes was measured using a streaming current electro kinetic analyzer (Anton Paar SurPASS, Austria) and 1 mM KCl was used as electrolyte solution at pH 7. In order to reduce the experimental error, the averaged value was measured by six different spots on each sample.

6

The PA selective layer was separated by dissolving the PSf support using dichloromethane. Raman analysis of selective layer was investigated using micro-Raman spectroscopy (Renishaw, UK). The thermal stability of selective layer was analyzed by thermos gravimetric analysis (TGA) with a DT-40 system. The obtained selective layer was heated from 0 to 800 °C under nitrogen atmosphere with a heating rate of 10 °C min-1. 2.5. Separation performance Permeability and selectivity of the prepared membranes was evaluated through a cross-flow RO testing device with three parallel filtration cells. The effective membrane area was 18.75 cm2 -1

-1

and the cross-flow velocity was 44.4 cm s . 2 g L NaCl feed solution was used to evaluate the RO performance. The membrane was compacted at 18 bar for 0.5 h and then measured at 16 bar. The concentrations of NaCl feed solutions and the collected permeate were measured using a conductivity meter. The water flux (J) and salt rejection (R) of the prepared membranes were measured using the following equations [30, 31]: (1) (2) where

,

and

represent the permeate volume, the effective area of the membrane

and permeate time, respectively, and

and

are the salt concentration in the feed and

permeate solution, respectively. In order to evaluate the durability of the prepared membranes, a long-term filtration test was performed at16 bar for 38 h and the feed solution was still a 2 g L-1 NaCl solution. The water flux and salt rejection were recorded every 1 h and then the intervals were extended to 2 h. 2.6. Antifouling ability To analyze the antifouling properties of the prepared membranes, fouling experiments were carried out with bovine serum albumin (BSA) and humic acid (HA) as foulant solution. The 7

filtration experiment continued for 3 h with 2 g L-1 NaCl solution as the feed solution to determine the initial flux. An almost constant water flux should be reached by adjusting pressure. Subsequently, 500 ppm BSA or HA was added in the NaCl solution. The fouling experiment lasted for 13 h. After BSA or HA filtration, DI water was used to clean the fouled membrane. After that, the membrane was irradiated under an ultraviolet lamp for 1 h and the recovered flux was measured using a 2 g L-1 NaCl solution for 3 h. The recovery rate (FRR) was calculated by the equation [32]: (3) where

is the recovered flux after irradiating and

is the initial flux.

2.7. Antibacterial capacity To evaluate the antibacterial properties of membrane, the growth of Escherichia coli (E. coli) on the membrane surfaces was investigated [33, 34]. E. coli bacterium cells were incubated in 25 mL of Luria-Bertani (LB) medium at 37 °C for 24 h. Then, the grown cells were centrifuged at 10000 rpm for 5 min and diluted to an appropriate concentration (approximately at 1.0×10 6 cfu -1

mL ) with sterilized water. Each sample film (3cm×3cm) was added to a conical flask and sterilized for 2 h under UV irradiation. Subsequently, the sample membranes were immersed into E. coli suspension, which were placed in an incubator at a constant temperature of 37 °C for 2 h. A group of membranes were illuminated under UV irradiation (365 nm) and the controlled membranes were placed in dark. Next, the membrane samples were rinsed with 5 mL of 1.0 wt % aqueous sodium chloride solution to collect E. coli cells. Then 0.1 mL of the collected bacterial suspensions without further dilution was spread onto a LB agar plate and incubated for 24 h at 37 °C. The bacteriostasis rate (

) was calculated using the following equation [35]: (4)

8

where

is the number of colonies on the plates contacting with each membrane sample and

is the number of colonies on the plates contacting with virgin TFC membrane. 3. Results and discussion 3.1. Pal/TiO2 nanocomposite characterization Fig. 2 presents the SEM and TEM images of the synthesized Pal/TiO2 (20/1) nanocomposites in comparison with the untreated Pal minerals. As shown in Fig. 2A and 2C, Pal exhibited a typical rod-like morphology with diameters of 30~60 nm and length of about 500~800 nm [18, 36]. It was evident that TiO2 particles were uniformly coated on the Pal rods and their size varied from 3~10 nm, which correlated well with the previous reported findings [29, 37]. A

B

C

D

Fig. 2. SEM images of (A) Pal, (B) Pal/TiO2(20/1) and TEM images of (C) Pal, (D) Pal/TiO2(20/1). The physical and chemical properties of Pal and Pal/TiO2 nanocomposite are described in Fig. 3. The effect of Pal/TiO2 mass ratio on the size and surface charge were analyzed by DLS measurement (Fig. 3A). Pal and Pal/TiO2 nanocomposites possessed similar size of approximately 500 nm, which was in good accordance with the TEM images. All the nanorods exhibited negative surface charges at pH=7. In addition, the zeta potential of Pal/TiO2 nanocomposite slightly decreased with coating more TiO2 on Pal surface.

9

Fig. 3B shows the XRD patterns of TiO2, Pal and Pal/TiO2(20/1). The TiO2 characteristic peaks at 25.3°, 37.7°, 47.6°, 53.9° and 54.8° could be exclusively indexed as their planes of (101), (004), (200), (105) and (211) [38]. The XRD patterns exhibited strong diffraction peaks at 25.3° and 47.6° indicating the anatase phase, which was preferred to other crystal structures because of its better photocatalytic activity [39, 40]. For Pal, the character peaks were found at 13.7°, 16.3°, 19.8°, 20.8° respectively, which were in consistent with the Si-O-Si crystalline layer [41]. The reflections at 2θ = 27.6° and 34.3° are corresponding to the (231) and (002) planes of Pal. Notably, XRD peaks of both Pal and TiO2 were observed in Pal/TiO2 nanocomposite, indicating that TiO2 grains were successfully deposited on Pal nanorods [42]. A

D

B

C

E

F

Fig. 3. (A) Size and zeta potential, (B) XRD patterns, (C) Raman spectra of Pal and Pal/TiO2(20:1) composite; (D) XPS survey spectra, (E) XPS O1s spectra and (F) XPS Ti2p spectra for Pal and Pal/TiO2(20:1) composite. Raman spectra for TiO2, Pal and Pal/TiO2(20/1) nanocomposite are presented in Fig. 3C. The Raman spectra of Pal/TiO2(20/1) exhibited both the single band at 140 cm−1 for Pal and the

10

characteristic peaks of TiO2 at 519 cm-1 (A1g), 399 cm-1 (B1g), and 144, 197, 639 cm-1 (Eg) [43]. It further documented that the Pal/TiO2 nanocomposite was synthesized. XPS was employed to reveal the chemical species of the elements of Pal, TiO2 and Pal/TiO2(20/1) nanocomposite within the near-surface region (Fig. 3D, 3E and 3F). Apparently, aluminum, silicon and magnesium species could be characterized by Al 2p, Si 2p, Si 2s and Mg 2p located at approximate 96 eV, 102 eV, 153 eV and 306 eV. As shown in Fig. 3E, Pal exhibited three O1s peaks positioned at 531.3, 532.4 and 533.8 eV, which can be assigned to COH, C=O and COOH, respectively. Apart from above-mentioned three O1s peaks, the peak at 530.1 eV could be assigned to Ti-O bond for Pal/TiO2. In the Ti 2p spectrum (Fig. 3F), the Ti 2p1/2 and Ti 2p3/2 spin-orbit splitted photoelectron states could be observed at 464.7 and 458.6 eV, which testified the existence of tetravalent titanium ions in the TiO2 lattice [44]. 3.2. Membrane characterization The morphology of the prepared membranes was investigated by SEM and AFM. Fig. 4 presents the SEM images of the surface of the TFC and TFN membranes. Both TFC and TFN membranes exhibited typical ridge and valley morphology, suggesting the successful formation of a PA layer via interfacial polymerization on PSf support [45, 46]. As presented in Fig. 4B and 4C, the incorporation of Pal or Pal/TiO2 induced larger nodular structures on the membrane surface. In addition, the cross-sectional images (Fig. 4 G-I) showed that the incorporation of Pal and Pal/TiO2 had negligible influence on the thickness of PA layer. The Pal clay and Pal/TiO2 nanocomposite possessed rich porosity and excellent hydration ability, which could improve the adsorption ability of water and MPD molecules as well as enlarge the interfacial region [20]. Comparing Fig 4E and 4F with 4D, Si, Mg and Al species appeared in the EDX image of the Pal embedded TFN membrane and Ti, Si, Mg and Al elements emerged in the EDX image of the

11

TFN75-Pal/TiO2(20/1). It demonstrated that the Pal and Pal/TiO2 were incorporated in the PA layer by interfacial polymerization. A B DA E F

B

C

D

E

F

G

H

I

Fig. 4. SEM images of the surface of the prepared membrane (A) TFC, (B) TFN75-Pal, (C) TFN75-Pal/TiO2(20/1) membranes, EDX images of the surface of the prepared membrane (D) TFC, (E) TFN75-Pal, (F) TFN75-Pal/TiO2(20/1), and cross-sectional images of (G) TFC, (H) TFN75-Pal, (I) TFN75-Pal/TiO2(20/1) membranes. To better investigate the morphology and roughness of the prepared membranes, the AFM analysis was applied. Fig. 5 shows the three-dimensional AFM images of both TFC and TFN membranes. Based on the root mean squared height (Sq) values, TFN membranes exhibited higher surface roughness than TFC membrane, which was resulted from the incorporated Pal and Pal/TiO2 nanorods as well as the larger nodular protrusions on the membrane surface. The higher surface roughness led to more contact area with water molecules, and hereby contributed to enhance the water flux.

A

B

C

12

Fig. 5. AFM images of (A) TFC, (B) TFN75-Pal, (C) TFN75-Pal/TiO2(20/1) membranes. The surface chemical properties of the prepared membranes were analyzed by ATR-FTIR and micro Raman. As shown in Fig. 6A, the characteristic peaks of the PSf support at 1585 cm-1 and 1488 cm-1, 1150 cm-1, 1242 cm-1, and 1294.5 cm-1 were ascribed to aromatic bands stretching, symmetric O=S=O stretching, asymmetric C–O–C stretching, and asymmetric O=S=O stretching vibrations respectively [47]. Both the TFC and the TFN membranes exhibited new appeared peaks at 1541 cm-1, 1610 cm-1 and 1660 cm-1, relating to C–N stretching vibration, C=O stretching vibration and C=O bands from amide group of the PA layer [48, 49]. It was noteworthy that a new band at 980 cm-1 assigned to Si–O–Si bond emerged in the FTIR spectra of the TFN75-Pal and TFN75-Pal/TiO2(20/1) membranes [50]. Moreover, the broad band at 3300 cm-1 was intensified, which further demonstrated the incorporation of Pal and Pal/TiO 2 nanorods with abundant hydroxyl groups in the PA layer. A

B

Fig. 6. (A) ATR-FTIR spectra of the prepared membranes and (B) Raman spectra of the PA selective layers.

13

Fig. 6B shows the Raman spectra of the PA selective layers separated from the TFC and TFN membranes. All the PA selective layers presented three characteristic peaks at 1074 cm-1, 1110 cm-1 and 1150 cm-1, corresponding to symmetric O=S=O stretching, antisymmetric O=S=O stretching and symmetric C–O–C stretching, because of the residual PSf adhered at the PA surface. The selective layer of the TFN-Pal membrane exhibited a peak at around 140 cm-1, which was ascribed to characteristic peak of the embedded Pal nanorods. For the TFN-Pal/TiO2 membrane, the anatase TiO2 characteristic peaks at 400 cm-1, 525 cm-1 and 628 cm-1 were observed. XPS data listed in Table 1 provided more evidence to confirm the incorporation of Pal and Pal/TiO2 in the PA layer. Si and Ti appeared in the XPS spectra of TFN75-Pal

Table 1. XPS results for TFC, TFN75-Pal and TFN75-Pal/TiO2(20/1) membranes. Membranes

C /%

O/%

N/%

Si/%

Ti/%

TFC

72.93

12.76

14.30

-

-

TFN-Pal

71.95

13.59

14.12

0.28

-

TFN-Pal/TiO2

71.82

13.74

14.08

0.21

0.05

The water contact angle curves are depicted in Fig. 7A. The contact angle decreased with the increase of incorporated Pal and Pal/TiO2, due to the existence of oxygen containing functional groups on the nanofillers [51]. As TiO2 nanoparticles contained more hydrophilic oxygenous groups, the TFN-Pal/TiO2 membrane showed higher hydrophilicity than the TFN-Pal membrane. The zeta potential data of the membrane surface are shown in Fig. 7B. All the membranes were negatively charged, and zeta potential became more negative by increasing the Pal and Pal/TiO2 contents. A

B

C

14

Fig. 7. (A) Water contact angle, (B) zeta potential of the prepared membranes and (C) TGA curves of the selective layers. Fig. 7C depicts the TGA curves of the selective layers separated from the TFC and TFN membranes. PA layer mainly decomposed at around 500 °C, accompanied by an evident weight loss [52]. The incorporation of Pal and Pal/TiO2 provided enhanced thermal stability for the selective layers, resulted from the excellent thermal stability of the clay and TiO2 nanofillers. Another possibility lay in the chemical interaction between the additives and the PA matrix. 3.3. Separation performance The performance of RO membranes is usually governed by water flux (J) and salt rejection (R). Fig. 8 reveals the RO results of the prepared TFC and TFN membranes. As shown in Fig. 8A, the TFC membrane exhibited a water flux of 24.5 l·m-2·h-1 (LMH) and a NaCl rejection of 98.2%. With the same test conditions, the commercial membrane SW30XLE showed a flux of 17.6 LMH and a NaCl retention rate of 98.2%. The permeate flux increased monotonously by increasing the Pal/TiO2 concentration in the TFN membranes. The salt rejection maintained at an acceptable level of 98%, when the concentration of Pal/TiO2 enhanced from 0 to 75 mg·l-1. However, more nanofillers (100 mg·l-1) induced significant decline in rejection. Similar trends were observed for the TFN-Pal membranes (Fig. S6) that the permeate flux enhanced without compromise in the selectivity when Pal concentration was lower than 75 mg·l-1. Addition of Pal and Pal/TiO2 could contribute to improve the TFN membranes’ roughness and hydrophilicity, which enlarged the contact area with water molecules and facilitated the water solubilization and diffusion trough the selective layer [53, 54]. Besides, the 0.37 × 0.63 nm2 sized tubular structure of Pal provided more high speed nanochannels for water transport [55]. In addition, some

15

nanocorridors could be induced between nanofillers and polyamide matrix, leading to fast pass of water molecules through the membranes. Appropriate amount of Pal and Pal/TiO2 (less than 75 mg·l-1) maintained a relatively integrated selective layer and then achieved a NaCl rejection similar to the TFC membrane. Further increase of Pal and Pal/TiO 2 concentration resulted in an aggregation of the nanorods in aqueous phase. Large particles brought severe steric hindrance and impeded the diffusion of MPD molecules. Therefore, the interfacial polymerization was suppressed and more defects were generated in the PA layer, leading to the significant decline of NaCl rejection. Overall, the permeability was influenced by the imperfect PA layer with sacrificing some selectivity at excessive Pal/TiO2 loading. A

B

Fig. 8. Water flux and NaCl rejection of the prepared TFN-Pal/TiO2 membranes (A) with different Pal/TiO2 concentration (Pal/TiO2=20/1) and (B) with different Pal/TiO2 mass ratio (Pal/TiO2 concentration=75 mg·l-1). Effect of the Pal/TiO2 mass ratio on membrane performance is shown in Fig. 8B. As the nanofillers was kept at a constant concentration (75 mg·l-1), the PA layer was well synthesized by interfacial polymerization without introducing extra defects. Consequently, the rejection could retain the similar value of 98% by varying the Pal/TiO2 mass ratio. An important phenomenon was that the permeability diminished with the increase of TiO2 proportion in the Pal/TiO2 nanocomposite. At the constant nanofiller concentration, Pal proportion decreased with increasing TiO2. It demonstrated that the tubular Pal worked as capillaries in the PA layer and

16

indeed provided extra water permselectivity for the TFN membranes. Kim and Kwak et al. reported that TiO2 modified membrane possessed a photocatalytic bactericidal effect [56]. Although the TFN75-Pal(1/0) membrane exhibited the highest flux, the TFN75-Pal/TiO2(20/1) membrane was selected for the subsequent antifouling and antibacterial tests. A 38 hour RO operation test was performed for each membrane to investigate the membrane stability. As shown Fig. 9, TFC, TFN75-Pal and TFN75-Pal/TiO2(20/1) membranes exhibited stable water fluxes and rejections during 38 h filtration, which indicated that robust PA layers were formed by incorporating appropriate Pal and Pal/TiO2 nanofillers. A

B

Fig. 9. Long-term separation performance of the TFC and TFN membranes as a function of time using 2 g/L NaCl solution at 16 bar and 25 °C.

3.4. Self-cleaning ability Membrane fouling is one of the main challenges that impair the performance of membranes during RO application. To evaluate the self-cleaning ability of the prepared membranes, the fouling-cleaning cyclic tests were performed employing BSA and HA as model protein foulant and natural organic foulant, respectively [57, 58]. Same initial flux of 24 LMH was applied for all antifouling tests to maintain an identical transverse hydrodynamic force for each membrane. As exhibited in Fig. 10A, the fluxes of all the membranes declined sharply after adding BSA in the feed solution and the fluxes partially recovered by DI water cleaning. Compared with TFC,

17

the TFN membranes gained lower pollution level and higher recovery flux because of the enhanced hydrophilicity and the increased negative surface charge. Both of them contributed to reduce the adsorption and promote the desorption of BSA and HA foulants [20]. After 1 h of UV irradiation, TFN75-Pal/TiO2(20/1) showed a clear improvement in water flux, while TFC and TFN75-Pal had no obvious change. This shows that the existence of TiO2 has a great contribution to the self-cleaning ability of the composite membrane. According to Fig. 10B, the FRR of Pal/TiO2(20/1) membrane increased from 76.8% to 87.0% after UV irradiation. However, the FRR of TFC and TFN75-Pal increased only slightly (FRR from 62.9% to 66.4% for TFC and from 79.3% to 83.7% for TFN75-Pal). This is because the free radicals produced by TiO2 under ultraviolet radiation damage the polymer matrix. From Fig. 10A and 10 C, similar fouling behaviors were observed by using HA as the foulant.

A

B

C

D

18

Fig. 10. The self-cleaning properties of the TFC, TFN75-Pal and TFN75-Pal/TiO2(20/1) membranes. (A) Time-dependent flux for BSA, (B) FRR of the prepared membranes for BSA before and after UV irradiation, (C) time-dependent flux for HA, (D) FRR of the prepared membranes for HA before and after UV irradiation. 3.5. Antibacterial capacity The antibacterial property of the membranes was measured by disk tests through evaluating the growth of E. coli. Fig. 11 shows the colonies of the E. coli bacteria on agar after contacting with the prepared membranes in dark and under UV illumination, respectively. The bactericidal rates of the membranes are shown in Table 2. The bactericidal rate of the TFC membrane was used as control to evaluate the antibacterial property of the TFN membranes. The TFN75-Pal and TFN75-Pal/TiO2(20/1) exhibited weak antibacterial rate of 12.2±7.3% and 13.2±6.0%, which could be attributed to the enhanced membrane hydrophilicity and the resulted poor affinity between the membrane surface and the bacteria [59, 60]. The bactericidal rates were significantly improved when the membranes were placed under UV illumination for 2h, which correlated well with the previously reported result [34]. Remarkably, the TFN75-Pal/TiO2(20/1) achieved a prominent bactericidal rate (Rb) of 98.2% with the assistance of UV illumination. The photocatalysis of TiO2 generated various active species, such as hydroxyl radical, hydrogen peroxide etc. These oxidative species could inhibit cell viability by destroying the outer membrane of bacteria [56]. Therefore, the introduction of Pal/TiO2 nanocomposite endowed the TFN membranes with enhanced permselectivity, advanced antifouling property and improved bactericidal capacity. A

B

C

D

E

F 19

Fig. 11. Antibacterial properties of the membranes shown on total agar plate counts. Representative photographs of the recultivated E. coli colonies on agar after contacting with (A) TFC, (B) TFN75-Pal and (C) TFN75-Pal/TiO2(20/1) in dark and after contacting with (D) TFC, (E) TFN75-Pal and (F) TFN75-Pal/TiO2(20/1) under UV illumination. Table 2. The bactericidal rates of E. coli for the TFC, TFN75-Pal and TFN75-Pal/TiO2(20/1) membranes. Membranes

in dark

under UV

TFC

0

62.8±7.8%

TFN-Pal

12.2±7.3%

69.2±6.4%

TFN-Pal/TiO2

13.2±6.0%

98.2±1.8%

Table 3 compares the RO performance, antifouling capacity and antibacterial activity of TFN membranes with different antibacterial constituents. The TFN membrane in this work exhibited excellent flux increase ratio. In addition, the TFN75-Pal/TiO2(20/1) possessed outstanding antifouling properties and antibacterial properties, which play an important role in membrane water treatment. Table 3. Comparison of the antifouling and bactericidal performance of TFN membranes with different antibacterial fillers. Antibacterial additives Functionalized GO[61]

J /LMH

R/%

Antifouling capacity

Antibacterial activity

36.3

95.3

200 ppm BSA, 7 days, Rt 40%

E. coli, Rb 94.1%

AgNPs[62]

38.2

98.9

—

E. coli, P. aeruginosa, S. aureus, 106 CFU -1 mL , Rb 100%

GO coated by tannic acid[63]

38.2

96.3

—

E. coli, 10 CFU mL , Rb 48%

CuNPs[64]

~45.0

99.3

200 ppm BSA, 10 h, Rt 23.9%, FRR 87.3%

E. coli, 106 CFU mL-1, Rb 99.0%

5

20

-1

8

AgNP@SiO2[65]

29.0

98.8

Pal (This work)

40.3

98.6

Pal/TiO2 (This work)

34.1

98.0

-1

E. coli, 10 CFU mL , Rb 92.1%; 6 -1 P. aeruginosa, 10 CFU mL , Rb 99.7%; 6 -1 S. aureus, 10 CFU mL , Rb 80.9%

— 500 ppm BSA, 21 h, Rt 39.7%, FRR 85.5%; 500 ppm HA, 21 h, Rt 33.6%, FRR 85.3% 500 ppm BSA, 21 h, Rt 35.9%, FRR 80.5%; 500 ppm HA, 21 h, Rt 37.9%, FRR 82.2%

E. coli, 106 CFU mL-1, under UV irradiation, Rb 69.2% E. coli, 106 CFU mL-1, under UV irradiation, Rb 98.2%

4. Conclusions This work has demonstrated that the incorporation of Pal/TiO2 nanocomposite into TFN membrane greatly increased the performance while offering opportunity to promote the permeability without compromise in selectivity and enhance the antifouling property as well as the photocatalysis bactericidal capacity. The contact angle test proved that Pal/TiO2 provided extra hydrophilicity for the TFN membranes, which contributed to improve both the permeability and the fouling resistance of the membranes. In addition, the embedded Pal with tubular structure worked as water channels to accelerate the water transport trough the dense PA matrix. Meanwhile, compared with TFC membrane and Pal incorporated TFN membranes, the TFNPal/TiO2 membrane exhibited excellent antimicrobial property under UV irradiation. The TFN75-Pal/TiO2(20/1) achieved a prominent bactericidal rate (Rb) of 98.2% with the assistance of UV illumination. As part of continuing efforts to further improve the performances of RO membranes, the current investigation paved a new way by the utilization of a multiple purpose nanocomposite material Pal/TiO2. Appendix A. Supplementary material. Supplementary data associated with this article can be found, in the online version, at FTIR spectra of TiO2, Pal and Pal/TiO2 nanocomposite; top view SEM images, cross sectional SEM images, AFM images and XPS spectra of the prepared membranes; separation performance of the TFN membranes with different Pal concentration.

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Graphical abstract

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